Robinson is a founder of the far-right English Defense League, which has organised protests against Muslim immigrants.

A diplomat in the United States has reportedly complained to the UK's ambassador in Washington, DC about the jailing of an English far-right activist.

Sam Brownback, the US ambassador for international religious freedom, raised the case of the activist known as Tommy Robinson in a June meeting with Sir Kim Darroch, Britain's ambassador to the US, Reuters news agency reported on Saturday, citing three unnamed sources, including a UK official.

Robinson is a founder of the English Defense League, a far-right group that has organised violent demonstrations against Muslim immigrants in the UK in the past decade.

More recently, Robinson, whose real name is Stephen Yaxley-Lennon, though he also uses other aliases, has styled himself as a journalist and campaigner against the threat of "Islamic extremism".

In late May, he was arrested outside a courthouse in the English city of Leeds while making video recordings about a trial related to child molestation. He received a 13-month jail sentence for violating English law limiting publicity during criminal trials.

Brownback raised the jailing of Robinson during a meeting with Darroch that covered a range of "religious freedom issues", the British official told Reuters earlier this week.

Brownback told Darroch that if Britain did not treat Robinson more sympathetically, the administration of US President Donald Trump might be compelled to criticise the UK's handling of the case, according to two sources in contact with organisers of aplanned pro-Robinson demonstration in the UK capital on Saturday.

The sources told Reuters that Robinson's supporters, who have also been in touch with the Trump administration about the issue, were concerned that he could be attacked by other prisoners.

Reuters was unable to determine why the top US official responsible for defending religious freedom would try to intervene with the British government on behalf of an activist who has expressed anti-Islamic views.

Brownback, who is a former governor of Kansas and former US senator, was not available for comment. However, on Thursday a US. State Department spokesman said the "characterisations" of Brownback's meeting with Darroch by Reuters sources were "completely false" but the spokesman did not elaborate further.

The British Embassy had no comment on further details of the discussion.

Last week, the Middle East Forum, a Philadelphia group, said it was sponsoring and organising a "Free Tommy Robinson" demonstration in London near the British Parliament on Saturday in collaboration with British and European groups.

The event was expected to merge with a demonstration in support of US President Donald Trump, who appointed Brownback, according to the British newspaper The Independent.

A spokesman for Hope Not Hate, a British anti-racism group, said: "In the week President Trump comes to the UK, his hand-picked diplomat allying himself with a far-right convicted fraudster perhaps shouldn't be too much of a shock."

On Friday, hundreds of thousands of people took to the streets of central London to protest against Trump's four-day visit to the UK.